In heat treatment processes (LTPS: Low Thermal Poly-Silicon, LITI: Laser Induced Thermal Imaging) for FPDs (Flat Panel Display: LCD, OLED, Flexible), an improvement in uniformity and productivity is required. In a LTPS process, oxide TFT technique is the matter of interest, instead of polysilicon crystallization TFT (Thin Film Transistor) using a heat source.
In general, a heat treatment process is applied to substrate heat oxidation and various annealing processes. Such a heat treatment process is performed using a heat source such as laser or tungsten halogen lamp. Currently, a heat treatment process using a laser heat source is applied in panels for flat panel displays (FPDs) including OLEDs (Organic Light Emitting Diodes). However, a heat treatment using a laser heat source allows for an instant heating of local region due to laser's nature, but there is a disadvantage that expensive laser generating equipments and complicate optical routes are required. Further, high maintenance costs are continuously needed.
For this reason, when using an oxide TFT substrate for panels of large area flat panel displays (FPDs), it has been suggested that a heat treatment should be performed using a tungsten halogen lamp used in conventional semiconductor heat treatment processes as a heat source for a heat treatment process. If a heat treatment of a substrate of a flat panel display panel is performed using a tungsten halogen lamp not laser, it is expected that more stable heat treatment would be performed in a large area substrate. However, when a heat treatment process for a large area substrate is performed using a tungsten halogen lamp as a heat source, there is a problem such as heat treatment uniformity, as exemplified in FIG. 3 and FIG. 4.
FIG. 1 shows a cross-sectional view of a heat treatment process using in line-type tungsten halogen lamps. There is a problem of uniformity that a substrate is subjected to ununiform temperature depending on an arrangement pattern of lamps 20. For a heat treatment of semiconductors, such a problem may be slightly relieved by rotating a wafer. However, a large area substrate cannot be rotated.
FIG. 2 shows a cross-sectional view of a heat treatment process using batch-type tungsten halogen lamps, which illustrates an indirect heating method through conduction from lamps 20. Likewise, for a batch-type heat treatment, there is a problem of uniformity that a substrate is subjected to ununiform temperature, because there is a temperature difference between an edge region adjacent to lamps 20 and a center region furthermost from lamps 20 within one substrate. Further, there is a problem that a temperature difference between substrates is generated depending on positions where substrates are situated (upper layer, medium layer, lower layer). Further, to solve such a problem of temperature uniformity, if a substrate is exposed to a heat source for a prolonged time, another problem that the substrate is subjected to thermal stress is caused.